Show him where the new litter is and take away the old one. I just recently changed the litter box's place from 2nd floor to 1st floor of my house and I showed my cat where I had placed it, and he never went anywhere else. Your situation should ne be very different. The tricky part might be to remove the box from the toilet once he gets used to going there.

Cats are extremely clean and will not go on the floor for no reason unless there are no other alternatives, if he knows there is a litter box at some other place, it should not be any different than if you would have changed the location of the litter box.